Electroluminescent ("EL") display devices are becoming increasingly popular, due in-part to cheaper fabrication and longer life provided by improvements in thin-film technology. Typically, the EL devices are formed of a number of transparent layers, including an EL layer which generates light when electricity flows through it. In addition to the EL layer, the devices also generally include a substrate and two electrodes (a cathode and an anode) on top of the substrate, with the EL layer situated between the electrodes. The EL layer can be formed from either inorganic or organic EL materials, each having its own chemistries, fabrication procedures, advantages and disadvantages.
As with other electronic display devices, EL devices can be made to be static or addressable. In the former case, a "static" EL device has a single display element which is turned "on" to display one image only; the image can be complex, meaning that it contains many visual attributes, but it is always turned "on" or "off" as a single unit, e.g., as a single pixel. In the latter case, an addressable EL device can include many rows and columns of single-pixel display elements, each of which typically represents a single visual attribute, and can be selectively controlled and illuminated so that the pixels collectively reproduce any desired image. EL devices of the latter type have drawbacks, however; often, it is difficult to fabricate or electrically connect the multiple rows and columns of display elements in a manner that they are close together and provide high resolution. Since many individual display elements usually make-up the display, brightness can vary widely between elements over time, since some pixels tend to be illuminated more frequently than others. Finally, it is expensive to fabricate these types of EL devices.
For these reasons, and due to relative simplicity of construction of a single element which unchangingly represents a complex image, static EL devices are sometimes best suited for use in many applications. As used herein, term "pixel" will be used to refer to the smallest addressable part of a display that can independently be switched "on" and "off." This disclosure relates to a static EL display element, that is, to a single pixel EL device which represents a complex image, having many visual attributes.
FIGS. 1A and 1B illustrate one prior art static device that displays an arbitrary image consisting of the re-entrant alphanumeric character ".RTM.." In particular, FIG. 1A shows a cross-section of a display device 41 which includes multiple thin-film layers, including a substrate 43, a first electrode 45, an EL layer 47 and a second electrode 49. The character ".RTM.," seen in FIG. 1B, is formed by patterning one of the electrodes 45 or 49, to have the shape of the character; in this manner, when the display is turned "on," electric current will flow through the EL layer 47 and generate light, but only in areas between the two electrodes 45 and 49, to illuminate the character ".RTM.."
In FIG. 1B, an electrode 51 is patterned during fabrication to represent multiple visual attributes of the desired image. Since the attributes are part of a single pixel display, each portion 53 of the image which is to be illuminated must overlie the electrode 51 and have a continuous electrical path to a terminal 57; for this reason, the electrode layer 51 is typically patterned in a manner that it is continuous across all of the illuminated portions 53 of the image; the electrode 51 connects them (a) together via a bridge 55 and (b) to the terminal 57 via a connecting path 59.
Again with reference to FIG. 1A, the particular one of the electrodes which is patterned (in the form of electrode 51) can be chosen to be either the first electrode layer 45 or the second electrode layer 49; in FIG. 1A, it is indicated to be the second electrode layer 45. Which ever electrode layer is chosen, patterning results in a display where undesired portions of the conductor, e.g., the bridge 55 and connecting path 59 of FIG. 1B, are necessarily always illuminated along with the character ".RTM." when the display device is switched "on." That is to say, the connecting bridges and paths of an image typically can not be eliminated from the display.
Selecting the first electrode layer 45 for patterning (the one closest to the substrate 43) typically requires a shadow mask or chemical etch procedure, which must be designed to provide a continuous electrode path, including the bridge 55 and connecting path 59. Since a patterned metal layer formed by a chemical etch or shadow mask procedure may tend to have sharp corners (not shown in the context of a first, patterned electrode layer, but designated 48 with respect to the second electrode layer 49 of FIG. 1A), use of a patterned first electrode layer 45 poses heightened device reliability problems (due to possible electrical shorting between electrodes 45 and 49) and requires special attention with respect to thickness and uniformity of the EL layer 47.
On the other hand, if the second, top electrode layer 49 of FIG. 1A is to be patterned, then this is typically performed during, or intermittent to, a vacuum deposition process of the EL layer 47 and the second electrode layer 49, since a shadow mask must be maneuvered into the deposition path. Practically speaking, insertion of a shadow mask is typically performed by interrupting a vacuum deposition process to insert the mask over the device 41 following deposition of the EL layer 47. Since individual thin film layers of the display device may also react adversely with air (forming undesired oxides), patterning of a second electrode layer generally also adds to complication, delay and expense of fabrication of the EL device, and detracts from its useful life. Also, as indicated in FIG. 1A, a patterned second electrode layer 49 typically leaves the EL layer exposed to air, and the second electrode layer 49 susceptible to water denigration by lateral seepage. The design factors just described also detract from large scale parallel device fabrication (using mass chip fabrication technologies, for example). For these reasons, conventional electrode patterning techniques present some significant drawbacks.
A definite need exists for a single element EL device which may be used to represent multiple attributes, yet which does not feature undesired illumination of conductor paths. Still further, a need exists for an EL device which may be used to present a smooth display with a very high level of resolution. Ideally, for example, such an EL device could be used to display photographic images. A need also exists for a means of creating a display which does not require interruption of a vacuum during the fabrication process, and hence, which is a simpler, more reliable process, and which features better economies of scale for simultaneous, parallel device fabrication. Finally, a need exists for a simpler, less expensive fabrication procedure. The present invention solves these needs and provides further, related advantages.